I. Introduction
This invention relates to electrolytic plating and more particularly, to a method of analyzing organic additives in an electroplating bath.
II. Discussion of Related Art
Electroplating is a complex process involving multiple ingredients in a plating bath. It is important that the concentration of several of the ingredients be kept within close tolerances in order to obtain a high quality deposit. In some cases, chemical analysis of individual solution constituents can be made regularly (such as pH measurement for acid content), and additions made as required. However, other addition agents such as brighteners, leveling agents, suppressants, etc., together with impurities, cannot be individually analyzed on an economical or timely basis by a commercial plating shop. Their operating concentration is low and their quantitative analysis is complicated and subject to error.
A prior art method for controlling such ingredients in an electroplating bath is to make regular additions of particular ingredients based upon empirical rules established by experience. However, depletion of particular ingredients is not always constant with time or with bath use. Consequently, the concentration of the ingredients is not actually known and the level in the bath eventually diminishes or increases to a level where it is out of the acceptable range tolerance. If the additive content goes too far out of range, the quality of the metal deposit suffers and the deposit may be dull in appearance and/or brittle or powdery in structure. Other possible consequences include low throwing power and/or plating folds with bad leveling.
Another prior art method for plating bath control is to plate articles or samples and visually evaluate the plating quality to determine if the bath is performing satisfactorily. In standard Hull Cell and "Bone Pattern" tests, a specially shaped test specimen is plated and then evaluated to determine the quality of the deposit along with shape. This is a time consuming test which gives at best a rough approximation of the concentration of the constituents of the bath.
The electroplating of through-hole interconnections in the manufacture of multilayer printed circuit boards is an example of the use of an electroplating metal where high quality plating is required. It is known that the concentration of the organic additives, such as brighteners and levelers, within the plating solution must be maintained in low concentration (typically less than 100 parts per million parts of solution - ppm) in order to obtain acceptable deposits on printed circuit boards. This must be done to maintain proper mechanical properties for resistance to thermal stresses encountered during manufacture and use and to assure the proper thickness of the deposit in the through-holes and leveling. The concentration of the organic additive agents fluctuates because of oxidation at the anode, reduction at the cathode, and chemical degradation. When the additive level is insufficient, deposits are burned and powdery in appearance whereas excessive addition agents induce brittleness and non-uniform deposition. Hull cell tests, Bone Pattern tests, and Pencil tests, combined with periodic additions of fresh additives, were the methods used to maintain a control concentration of the additive until recently. These methods were unreliable and circuit board quality suffered as a consequence of these unreliable methods.
A more recent method for evaluating the quality of an electroplating bath was disclosed in Tench U.S. Pat. No. 4,132,605 (hereafter the Tench patent). In accordance with the procedures of the Tench patent, the potential of a working electrode is swept through a voltammetric cycle, including a metal plating range and a metal stripping range, for at least two baths of known plating quality and an additional bath whose quality or concentration of brightener is to be evaluated. The integrated or peak current utilized during the metal stripping range is correlated with the quality of the bath of known quality. The integrated or peak current utilized to strip the metal in the bath of unknown quality is compared to the correlation and its quality evaluated. In a preferred embodiment of said patent, the potential of an inert working electrode is swept by a function generator through the voltammetric cycle. A counter electrode immersed in the plating bath is coupled in series with a function generator and a coulometer to measure the charge from the working electrode during the stripping portion of the cycle.
An improvement to the method disclosed in the Tench patent is described by Tench and White, in the J. Electrochem. Soc., "Electrochemical Science and Technology", April, 1985, pp. 831-834 (hereafter the Tench publication). In accordance with the Tench publication, contaminant buildup in the copper plating bath affects the copper deposition rate and thus interferes with brightener analysis. The Tench publication teaches that rather than continuous sweep cycle utilized in the above-referenced patent, a method be used involving sequentially pulsing the electrode between appropriate metal plating, metal stripping, cleaning, and equilibrium potentials whereby the electrode surface is maintained in a clean and reproducible state. Stated otherwise, where the process of the Tench patent involves a continuous voltammetric sweep between about -600 mV and +1,000 mV versus a working electrode and back over a period of about 1 minute, the Tench publication pulses the potential, for example at -250 mV for 2 seconds to plate, +200 mV for a time sufficient to strip, +1,600 mV to clean for seconds, +425 mV for 5 seconds to equilibrate, all potentials referenced to a saturated Calomel electrode, after which the cycle is repeated until the difference between successive results are within a predetermined value, for example, within 2% of one another.
The procedure of the Tench publication provides some improvement over the procedure of the Tench patent, but during continuous use of an electroplating bath and following successive analysis, contaminants build up on the electrodes and analysis sensitivity is lost.
An improvement over both the Tench patent and the Tench publication is found in U.S. Pat. No. 4,917,774. In accordance with the invention, in order to prevent contaminant buildup on the electrodes, a pause without applied potential is used following each completed cycle. This is effectuated by an open circuit condition or an applied potential equal to or approximately equal to the open circuit potential of the inert electrode in the bath following the cycle of metal plating, metal stripping, and if a pulse system is used, cleaning. An applied potential equal to or approximately equal to the open circuit potential can be applied in lieu of an equilibration step, or an open circuit condition can be used following equilibration. During either this applied potential or the open circuit condition, contaminants are either eliminated from the electrode surface or fail to deposit on the surface. The method involves passing an inert electrode through a predetermined sequence of voltammetric steps including a metal plating step, a metal stripping step, and a conditioning step without applied potential, correlating quantity of brightener with the coulombs utilized during the metal stripping step, and using the same predetermined sequence of voltammetric steps for a bath having an unknown quantity of additive.
All of the prior art methods for evaluating the quality of electroplating baths are based on various voltammetric cycles involving metal plating and stripping. The prior art methods are also cyclic, meaning that the entire process has to be repeated more than once before a final determination of the level of brightener is made. The quantity of metal deposited during the metal plating cycle and subsequently redissolved in the metal stripping cycle, is related to the concentration of brightener affecting the rate of deposition. The methods observe the current density of copper ions reducing on an electrode at a given potential. This can then be related to brightener concentration since brightener will increase the current density. These voltammetric methods may be used to analyze the brightener content, content of organic contamination and the chloride content.
The method of the present invention differs from conventional cycled voltammetric methods. The concentration of leveling agents and brighteners in an electroplating bath induces leveling of the deposit by inhibiting deposition at peaks, where the concentration remains high, and enhancing deposition in recesses, where the concentration becomes depleted as it is diffusion limited. It has been found that the brightness and leveling phenomena is related to diffusion controlled adsorption of these components. In the present invention, the quantity of these additives in the electroplating bath is measured directly.
The use of a direct method of measuring such components is not taught in the prior art and the use of such methods is discouraged. For example, U.S. Pat. No. 4,132,605, col. 3, lines 12-21, states that conventional voltammetric analysis in which the substance to be determined is, itself, adsorbed and then stripped from the electrode surface, is not a suitable method for determining the quantity of low concentration additives in a plating bath because the quantity of the additive cannot be separated from the large quantity of metal being deposited.